The present invention is directed to an improved air press system which utilizes a plurality of air plenums above a belt for holding the belt on a corrugated board as it passes over heating elements to dry the board.
In drying double-back corrugated boards during a gluing process, it was known to use a plurality of ballast rolls to apply pressure on the board as it was being passed over the heating elements, such as steam chests. A problem with this procedure is that the rolls would sometimes crush the corrugated board so that it would have a reduced height of between 0.01" and 0.02" and the crushing would also result in a loss of strength for the board.
To overcome the problems of crushing, it has been suggested to utilize a plurality of air plenums which were positioned over a double-backer belt to apply pressure on the board as it moved over the steam chests. These plenums were carefully adjusted to provide a small gap between the peripheral edge and a double-back belt and, thus, when changing the thickness of the double-backed corrugated board, the plenums would need to be raised and lowered by a corresponding amount.
Other problems occur in such a device, in addition to the time required for setting it up for a change in thickness for the corrugated board being processed. For example, the device did not apply sufficient pressure to certain portions of the corrugated sheet, while it applied excessive pressure to other portions. In addition, since each of the plenums was provided with the edges extremely close to the board, which is in a form of a web, to minimize air loss, the change of air in each plenum was substantially less and this air had a tendancy to have excessive moisture after the drying operation had started.